Transparent conducting oxide (tco) based integrated modulators

ABSTRACT

A photonic waveguide assembly has a photonic waveguide for transmission of light or a substrate, and an optical refractive index modulator positioned about said photonic waveguide to modulate the phase or amplitude, or combination thereof of the light traveling in the photonic waveguide, or transmitting through or reflecting of the substrate.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/719,988, filed Aug. 20, 2018, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Indium tin oxide (ITO) is a ternary compound which belongs to the classof transparent conductive oxide (TCO). For its coexisting optical andelectrical properties, the large interest surrounding ITO, which wasmainly pushed due to its use in the industry, product-related purposesand timely applications, enabled high-yield and reliable wafer scalefabrication processes, compatible with the CMOS technology productionline, which makes this material even more appealing for a plethora ofother applications. The underlying modulation mechanism for all TCOmaterials (e.g. aluminum doped zinc oxide (AZO), fluorine doped tinoxide (FTO), gallium-doped zinc oxide (GZO), indium-doped zinc oxide(IZO), magnesium-doped zinc oxide (MZO), aluminum and gallium co-dopedzinc oxide (AGZO), Indium gallium zinc oxide (IGZO), Zinc Oxide (ZnO),Indium oxide (In₂O₃) etc.), including ITO, are the same—free carrierabsorption dynamics arising from accumulation/depletion of the carriersin a capacitive stack. As the drive voltage is increased, free carriersleading to dispersive effects are induced and a corresponding netincrease occurs in the carrier concentration.

Electro-optic (EO) modulators can be used, for example, to convertbetween electrical signals (e.g., digital signals) and optical signals.EO modulation is achieved by either changing the real part (n) of themodal refractive index leading to phase shifting-based interferometricdevices termed electro-optic modulators (EOM), or by modulating theimaginary part (c) of the modal index of linear electro-absorptivemodulators (EAM). In both types, the fundamental complex index ofrefraction is altered electrically in the active material, which in turnmodifies the propagation constant of the mode inside the respectivewaveguide. EOMs operate by changing the real part of the index, whichrelates to the phase of the light, whereas EAMs operate by changing theimaginary part of the index, which relates to the intensity absorptionof the light.

Kramers-Kronig relations dictate that changing the real part of thecomplex index independent from simultaneously altering the imaginarypart is impossible. Since phase modulators require interferometricschemes (e.g., ring resonators and Mach Zehnder Interferometers (MZI)),they do inherently suffer from an extended footprint compared toabsorption-based modulators. In Mach Zehnder Modulators (MZM) (which isan MZI used in an active modulation scheme), the product of thehalf-wave voltage times the active modulator length, V_(π)L, is a figureof merit (FOM) since they exhibit a tradeoff between obtaining π-phaseshift with increased device length or voltage.

The devices exhibiting advanced FOMs, shown in Table 1 below, amount toplasmonics, integration of organic/polymer materials, III-V quantum wellstructures, etc. Many of these schemes essentially offer acceptableperformance but are mostly difficult to integrate in the mature Siprocess. The present invention can avail ease of fabrication and CMOSintegration. The references noted are herein incorporated by reference.

TABLE 1 Figure of merit (FOM) comparison for Mach Zehnder devices withdifferent active modulation materials and waveguide structures in recentyears V_(π)L Structure/Material (V. μm) Ref. Si Wrapped 140,000 F. Y.Gardes, D. J. Thomson, N. G. Emerson, and G. T. Reed, around-pn “40 Gb/ssilicon photonics modulator for TE and TM polarisations,” Opt. Express19(12), 11804-11814 (2011). Coplanar 120,000 E. L. Wooten, K. M. Kissa,A. Yi-Yan, E. J. Murphy, D. A. Lafaw, waveguide P. F. Hallemeier, D.Maack, D. V. Attanasio, D. J. Fritz, LiNbO₃ G. J. McBrien, and D. E.Bossi, “A review of lithium niobate modulators for fiber-opticcommunications systems,” IEEE J. of Select. Topics in Quant. Elec. 6(1),69-82 (2000). Si Wrapped 110,000 F. Y. Gardes, D. J. Thomson, N. G.Emerson, and G. T. Reed, around-pn “40 Gb/s silicon photonics modulatorfor TE and TM polarisations,” Opt. Express 19(12), 11804-11814 (2011).Domain inverted 90,000 F. Lucchi, D. Janner, M. Belmonte, S. Balsamo, M.Villa, S. Giurgiola, push-pull LiNbO3 P. Vergani, and V. Pruneri, “Verylow voltage single drive domain inverted LiNbO₃ integrated electro-opticmodulator,” Opt. Express 15(17), 10739-10743 (2007). Dual driven 80,000E. L. Wooten, K. M. Kissa, A. Yi-Yan, E. J. Murphy, D. A. Lafaw,coplanar P. F. Hallemeier, D. Maack, D. V. Attanasio, D. J. Fritz,waveguide G. J. McBrien, and D. E. Bossi, “A review of lithium niobateLiNbO₃ modulators for fiber-optic communications systems,” IEEE J. ofSelect. Topics in Quant. Elec. 6(1), 69-82 (2000). Si Vertical-pn 40,000L. Liao, A. Liu, D. Rubin, J. Basak, Y. Chetrit, H. Nguyen, R. Cohen, N.Izhaky, and M. Paniccia, “40 Gbit/s silicon optical modulator for highspeed applications,” Electron. Lett. 43(22), 1196-1197 (2007). BulkLiNbO₃ 36,000 D. Janner, D. Tulli, M. Garcia-Granda, M. Belmonte, and V.Pruneri, physical limit “Micro-structured integrated electro-opticLiNbO₃ modulators,” Laser Photonics Rev. 3(3), 301-313 (2009). Si pipin35,000 M. Ziebell, D. Marris-Morini, G. Rasigade, J.-M. Fédéli, P.Crozat, E. Cassan, D. Bouville, and L. Vivien, “40 Gbit/s low- losssilicon optical modulator based on a pipin diode,” Opt. Express 20(10),10591-10596 (2012). Si Lateral-pn 28,000 D. Thomson, F. Gardes, J.Fedeli, S. Zlatanovic, Y. Hu, B. Kuo, E. Myslivets. N. Alic, S. Radic,G. Z. Mashanovich, and G. T. Reed, “50 Gbit/s silicon opticalmodulator,” IEEE Photon. Technol. Lett. 24(4), 234-236 (2012). SiLateral-pn 27,000 D. J. Thomson, F. Y. Gardes, Y. Hu, G. Mashanovich, M.Fournier, P. Grosse, J.-M. Fedeli, and G. T. Reed, “High contrast 40Gbit/s optical modulation in silicon,” Opt. Express 19(12), 11507-11516(2011). Si pn-depletion 24,000 L. Chen, C. R. Doerr, P. Dong, and Y.-K.Chen, “Monolithic silicon chip with 10 modulator channels at 25 Gbps and100-GHz spacing,” Opt. Express 19(26), B946-B951 (2011). Dopingoptimized 20,500 X. Xiao, H. Xu, X. Li, Z. Li, T. Chu, Y, Yu, and J. Yu,“High- Si speed, low-loss silicon Mach-Zehnder modulators with dopingoptimization,” Opt. Express 21(4), 4116-4125 (2013). Si Self-aligned-pn18,600 P. Dong, L. Chen, and Y.-K. Chen, “High-speed low-voltagesingle-drive push-pull silicon Mach-Zehnder modulators,” Opt. Express20(6), 6163-6169 (2012). Integrated thin 18,000 C. Wang, M. Zhang, B.Stern, M. Lipson, and M. Loncar, film LiNbO₃ on “Nanophotonic lithiumniobate electro-optic modulators,” Opt. insulator Express 26(2),1547-1555 (2018), Si pin 13,000 S. Akiyama, T. Baba, M. Imai, T.Akagawa, M. Noguchi, E. Saito, Y. Noguchi, N. Hirayama, T. Horikawa, andT. Usuki, “50-Gbit/s silicon modulator using 250-_m-long phase shifterbased on forward-biased pin diodes,” in Proceedings of 9 ^(th) IEEEInternational Conference on Group IV Photonics (IEEE, 2012), pp.192-194. Silicon-organic 9,000 L. Alloatti, D. Korn, R. Palmer, D.Hillerkuss, J. Li, A. Barklund, hybrid (SOH) R. Dinu, J. Wieland, M.Fournier, J. Fedeli, H. Yu, W. Bogaerts, P. Dumon, R. Baets, C. Koos, W.Freude, and J. Leuthold, “42.7 Gbit/s electro-optic modulator in silicontechnology,” Opt. Express 19(12), 11841-11851 (2011). Si Lateral-pn8,500 A. Brimont, D. J. Thomson, F. Y. Gardes, J. M. Fedeli, G. T. Reed,J. Mart'1, and P. Sanchis, “High-contrast 40 Gb/s operation of a 500 μmlong silicon carrier-depletion slow wave modulator,” Opt. Lett. 37(17),3504-3506 (2012). Si Projection 5,000 J. Fujikata, J. Ushida, T.Nakamura, Y. Ming-Bin, Z. ShiYang, MOS D. Liang, P. L. Guo-Qiang, and D.Kwong, “25 GHz Operation of Silicon Optical Modulator with ProjectionMOS Structure,” in Optical Fiber Communication Conference, OSA TechnicalDigest (CD) (Optical Society of America, 2010), paper OMI3. III-VMultiple 4,600 S. Dogru and N. Dagli, “0.77-V drive voltageelectro-optic Quantum Wells modulator with bandwidth exceeding 67 GHz,”Opt. Lett. (MQW) 39(20), 6074-6077 2014). SOH 3,800 R. Palmer, L.Alloatti, D. Korn, P. C. Schindler, M. Baier, J. Bolten, T. Wahlbrink,M. Waldow, R. Dinu, W. Freude, C. Koos, and J. Leuthold, “Low powerMach-Zehnder modulator in silicon-organic hybrid technology,” IEEEPhotonics Technol. Lett. 25(13), 1226-1229 (2013). GaAs/AlGaAs 2,100 J.Shin, Y.-C. Chang, and N. Dagli, “0.3 V drive voltage GaAs/AlGaAssubstrate removed Mach-Zehnder intensity modulators,” Appl. Phys. Lett.92, 201103 (2008). Hybrid Si MQW 2,000 H.-W. Chen, Y. Kuo, and J. E.Bowers “Hybrid silicon modulators,” Chin. Opt. Lett. 7(4), 280-285(2009). InGaAlAs/InAlAs 600 S. Dogru and N. Dagli, “0.2 V drive voltagesubstrate removed MQW electro-optic Mach-Zehnder modulators with MQWcores at 1.55 μm,” J. Lightwave Technol. 32(3), 435-439 (2014). ITO MOS520 R. Amin, R. Maiti, C. Carfano, Z. Ma, M. H. Tahersima, Y. Lilach,(Photonic ITO D. Ratnayake, H. Dalir, and V. J. Sorger, “0.52 V mm MZI -This ITO-based Mach-Zehnder modulator in silicon photonics,” invention)*APL Photonics 3(12), 126104 (2018). Si P⁺-i-n⁺ 360 W. M. J. Green, M. J.Rooks, L. Sekaric, and Y. A. Vlasov, “Ultra-compact, low RF power, 10Gb/s silicon Mach-Zehnder modulator,” Opt. Express 15(25), 17106-17113(2007). ITO Plasmonic 95 N/A Vertical MOS (Plasmonic ITO MZI VerticalCapacitor - This invention)* EO Polymer 70 F. Li, M. Xu, X. Hu, J. Wu,T. Wang, and Y. Su, “Monolithic Plasmonic silicon-based 16-QAM modulatorusing two plasmonic phase shifters,” Opt. Commun. 286, 166-170 (2013).ITO Lateral MOS 63 N/A (Plasmonic ITO Lateral Capacitor MZI - Thisinvention)* Liquid crystals 60 C. Haffner, W. Heni, Y. Fedoryshyn, J.Niegemann, A. Melikyan, with SOH slot/ D. L. Elder, B. Baeuerle, Y.Salamin, A. Josten, U. Koch, all-plasmonic C. Hoessbacher, F. Ducry, L.Juchli, A. Emboras, D. Hillerkuss, polymer M. Kohl, L. R. Dalton, C.Hafner, and J. Leuthold, “All plasmonic Mach-Zehnder modulator enablingoptical high-speed communication at the microscale,” Nat. Photonics 9,525-528 (2015).

SUMMARY OF THE INVENTION

It is one object of the invention to provide a photonic waveguide thatmodules the optical refractive index of light traveling in thewaveguide. In accordance with this and other objectives, a photonicwaveguide assembly has a photonic waveguide for transmission of light ora substrate, and an optical refractive index modulator positioned aboutsaid photonic waveguide to modulate the phase or amplitude, orcombination thereof of the light traveling in the photonic waveguide, ortransmitting through or reflecting of the substrate. The photonicwaveguide assembly can be, for example, and MZI.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) shows an MZI in accordance with the invention;

FIG. 1(b) is an optical microscope image of the device of FIG. 1(a);

FIG. 1(c) is a Scanning Electron Microscope (SEM) image of the device ofFIG. 1(a) and the inset shows a Focused Ion Beam (FIB) cross-section ofthe active capacitor;

FIG. 1(d) is Finite Element Method (FEM) Eigenmodes of the active devicecorresponding to the ON and OFF states of modulator operation;

FIG. 2(a) is a graph of I-V measurements;

FIG. 2(b) is a graph of the transmission spectra;

FIG. 2(c) is a graph of the normalized output optical power vs. drivevoltage;

FIG. 2(d) is a graph of the extracted effective index change and ITOmaterial index change vs. drive voltage;

FIG. 3(a) shows balancing loss by depositing Au on the passive MZI armto maximize the Extinction Ratio (ER);

FIG. 3(b) shows the ER maximization technique with Y-junction splittingoptimization where the Insertion Loss (IL) is minimized;

FIG. 4(a) a lateral MOS ITO based MZI in accordance with the invention;

FIG. 4(b) shows an optical microscope image of the MZI of FIG. 4(a);

FIG. 4(c) shows cross-section views of the device of FIG. 4(a);

FIG. 5(a) is a graph of I-V measurements;

FIG. 5(b) is a graph of normalized output optical power vs. biasvoltage;

FIG. 5(c) shows FEM simulation with electrostatic field overlap;

FIG. 5(d) shows field line interaction with the ITO layer, a magnifiedview of FIG. 5(c) in the area of interest;

FIG. 6(a) is cross-section views of a vertical MOS ITO based MZI arms;

FIG. 6(b) is a vertical MOS ITO based MZI in accordance with theinvention;

FIG. 6(c) is top view schematic diagram of the FIG. 6(b) with contactsshown;

FIG. 6(d) is an optical microscope image of the MZI of FIG. 6(b);

FIG. 6(e) is a graph of normalized output power vs. bias voltage, andI-V measurements;

FIG. 6(f) shows FEM eigenmodes for differently polarized modescorresponding to the ON and OFF states of modulator operation;

FIG. 7(a) shows a 3D perspective view of the vertical slot ITO PCNBcavity with air holes;

FIG. 7(b) is a top view of FIG. 7(a);

FIG. 7(c) is a side view, longitudinal cross-section, and thecross-sectional structure is shown as an inset;

FIG. 7(d) is an electric field intensity distribution profile;

FIG. 7(e) is a graph of transmission intensity vs. wavelengthcharacterizing the resonance shift in the cavity in the inset;

FIG. 7(f) is a graph manifesting Q-factor and Purcell factorenhancements over similar prior art;

FIG. 8(a) is a cross-section schematic of a ITO/oxide/Graphene capacitorenabled absorption modulator in accordance with the invention;

FIG. 8(b) is an optical microscope image of the ITO/oxide/Grapheneabsorption modulator in FIG. 8(a);

FIG. 8(c) is a graph of ER vs. bias voltage;

FIG. 8(d) is a graph of I-V measurements of the modulator in FIG. 8(b);and

FIG. 8(e) is a graph of output power vs. bias voltage.

DETAILED DESCRIPTION OF THE INVENTION

In describing the illustrative, non-limiting embodiments of theinvention illustrated in the drawings, specific terminology will beresorted to for the sake of clarity. However, the invention is notintended to be limited to the specific terms so selected, and it is tobe understood that each specific term includes all technical equivalentsthat operate in similar manner to accomplish a similar purpose. Severalembodiments of the invention are described for illustrative purposes, itbeing understood that the invention may be embodied in other forms notspecifically shown in the drawings.

Transparent conductive oxides (TCOs) are able to highly tune theiroptical properties with applied bias in capacitor configurations viaaltering their free carrier concentration and hence plasma dispersion.The TCO material indium tin oxide (ITO) exhibits unity-strong indexchange and epsilon-near-zero behavior. The choice of gatingconfiguration and confinement factors can render different optical modesfor modulation efficacy. Different optical modes, i.e.photonic/plasmonic/hybrid-plasmon-polariton (HPP), can be employed incapacitive stacks to invoke carrier injection (removal) leading todispersive effects causing the free carrier accumulation (depletion)based modulation.

This capacitive gating strategy to tune the carrier concentration, andhence, the complex refractive index of the TCO can accomplish both phaseand absorption modulation simultaneously. Phase modulation alwaysnecessitates a reference phase for comparison with the induced alteredphase and as such requires some sort of interferometric arrangement(e.g. cavity feedback, travelling wave interferometers, etc.).Absorption modulators can be implemented in rather simple schemes (e.g.linear waveguides). Optical waveguide modulator modes can have: a)different polarizations; such as Transverse Electric (TE) or TransverseMagnetic (TM), and b) different structural mechanisms; such as photonicor surface-plasmon-based mode, or hybrid of photon/plasmon polaritonic(HPP) modes. The present invention demonstrates TCO based modulation inintegrated Si photonics for both phase and absorption modulation inseveral schemes as manifested in Table 2 below. Capacitive modulationenabled stacks of a TCO material and another resistive material such asmetal, dielectric or doped dielectric, TCO material, 2D materials, orgel-like materials can be utilized in such schemes.

TABLE 2 Different Electro-optic modulators formulated by transparentconducting oxide (TCO) capacitive stacks Modulator Optical Type FeedbackMode Capacitor Structure (Gating scheme) EOM (Phase MZI PhotonicSemiconductor (e.g. Si)/Oxide (e.g. SiO₂, Al₂O₃, modulation) etc.)/TCO(e.g. ITO) Plasmonic Metal (e.g. Au)/Oxide (e.g. SiO₂, Al₂O₃, etc.)/ TCO(e.g. ITO) Lateral Capacitor Metal (e.g. Au)/Oxide (e.g. SiO₂, Al₂O₃,etc.)/ TCO(e.g. ITO) Vertical Capacitor (HPP) PhC Photonic Semiconductor(e.g. Si)/Oxide (e.g. SiO₂, Al₂O₃, etc.)/TCO (e.g. ITO)/Oxide (e.g.SiO₂, Al₂O₃, etc.)/Semiconductor (e.g. Si) EAM N/A Photonic TCO (e.g.ITO)/Oxide (e.g. SiO₂, Al₂O₃, etc.)/ (Absorption 2D Material (e.g.Graphene) modulation) TCO (e.g. ITO)/Oxide (e.g. SiO₂, Al₂O₃, etc.)/ TCO(e.g. ITO)

All the different optical modes and capacitive gating schemes listed inthis Table 2 can be used for both phase and absorption modulation and inconjunction with any feedback system also, not just limited to thistable (e.g. ring resonators, Fabry-Perot cavities, coupledwaveguides/resonators, etc.). All these schemes can be utilized inPhotonic Integration Circuits (PICs) platforms including siliconphotonics, silicon nitride photonics, III-V based photonics,polymer-based waveguide structures, any oxide or nitride based waveguideplatform such as SiO₂ for example, or any-other material forming awaveguide including fiber-based structures.

Photonic ITO MZI

Turning to the drawings, FIG. 1 shows an ITO-based Mach ZehnderInterferometer (MZI) 100 having an active gating region and contacts. Anoptical MZI is shown, for example, in U.S. Patent Publication No.2019/0072833, which is hereby incorporated by reference. Referring toFIG. 1(a), the MZI splits into two arms, each forming a photonicwaveguide for the transmission of light. A first arm has the bottomelectrode 102 to the Si waveguide for capacitive gating, lead line 104,and contact pad 106, all of which are formed of gold (Au) to make itlossy and offset optical losses. The capacitor bottom contact 102contacts at least a portion of the first arm. The lead line 104 extendsbetween the capacitor 102 and the contact pad 106, which is connected toground. The metal (Au) on the un-modulated arm serves as a contact 106for the capacitor while adding necessary loss for balancing the device.FIG. 1(b) shows an optical microscope image of the fabricated deviceshowing the active modulation region and contacts. The dashed whiteoutline marks the patterned 10 nm ITO thin film designating the activedevice region on top of the corresponding Si waveguide (activelymodulated arm of the MZM).

As further shown in FIG. 1(a), the second arm of the MZI 100 includes asecond electrode 112, for example a phase modulator on an Al₂O₃ gateoxide 114, phase modulator contact or contact pad 116, and an Au contactor contact pad 118 for the MOS stack. The phase modulator 112 can be anysuitable element that modulates the phase of light traveling through thearm waveguide. In the current example embodiment, the phase modulator isan optical index modulator that changes the index of refraction of lighttraveling through the arm waveguide with electrostatic gating, and isspecifically shown with the active material ITO. For example, a voltagecan be used to control the capacitive charging/discharging of the phasemodulator 112, which in turn changes the optical index in the waveguide(i.e., the effective index of refraction of the waveguide changes),which in turn changes the phase of the light traveling in the armwaveguide. Thus, (Si) forms a first electrode and the ITO phasemodulator 112 forms a second electrode for the capacitor 102.

The light in each arm are in-phase when the loss in each arm is equal sothat the light combines where the arms join and provide the MZI output.If the light in each arm are out-of-phase, there will be a partial orcomplete loss of light when the arms combine.

The gate oxide 114 contacts and covers or surrounds at least a portionof the second arm, and the ITO 112 contacts and covers or surrounds atleast a portion of the gate oxide 114. A lead line extends between theITO and the ITO contact pad 116. The contact 118 contacts and is on topof the ITO contact pad 116, and a drive voltage V_(d) is applied to thecontact 118. The Au contact 118 is on top of the ITO contact 116, asshown. The etched oxide opening on top of the metal pad 108 (FIG. 1(b))on the bottom Si contact 106 can be noticed from the color contrastdifference. As further shown, the contacts 106, 104, 102, gate oxide 114and ITO layer 112 can be formed from thin layers.

Two metal contact pads 106, 118 side by side are deposited on bothcontact areas to facilitate biasing (marked with Au). The ITO contact116 is used to administer the voltage, while the bottom Si contact 106is grounded. TM-optimized grating couplers are used to couple the lightfrom (to) the fiber into (out of) the MZM. Relevant parameters arelength of the metal contact on Si, L_(m)=3.7 μm; active device length,L=32 μm; thickness of the deposited ITO thin film, t_(ITO)=10 nm:thickness of the Al₂O₃ gate oxide, t_(ox)=10 nm; waveguide height, h=220nm; and width, w=500 nm.

Turning to FIG. 1(c), a Scanning Electron Microscope (SEM) imageillustrates the fabricated device showing the deposited ITO thin filmover the Si waveguide in the active modulation region. Atomic LayerDeposited (ALD) Al₂O₃ is everywhere on top of the waveguide andunderneath the ITO film. A focused ion beam (FIB) SEM cross section ofthe active device revealing the oxide/ITO capacitive stack on top of theSi waveguide is shown as an inset.

FIG. 1(d) depicts the finite element method eigenmodes for the activeregion cross-section (inset, FIG. 1(c)) for the ON and OFF states ofoperation showcasing the electric-field in the cross-sectional structurerevealing the effective light confinement in the arm waveguide underelectrical bias.

FIG. 2(a) shows I-V measurements of the fabricated device. ASavitzky-Golay smoothing function has been applied on the measured datato showcase the I-V characteristics of the device. FIG. 2(b) shows thetransmission spectra (in dB) of the device for varying drive voltagesvs. wavelength, λ (nm). The transmission includes all the passivecomponent losses including the grating couplers, Y-junctions, bentwaveguides, etc. FIG. 2(c) shows normalized output optical power,P_(out) (a.u.), vs. drive voltage, V_(d) (Volts); the dashed linerepresents a squared sinusoidal fit [cos²(arg)] suggested by theunderlying physics of the MZ scheme. FIG. 2(d), top, shows extractedeffective index change, Δn_(eff), and FIG. 2(d), bottom, shows ITOmaterial index change, Δn_(ITO), with applied bias, V_(d) (Volts),corresponding to modulation effects; the ITO index observes near unityorder change with applied bias.

FIG. 3 shows extinction ratio, ER, and insertion loss, IL performancefor two different loss-balancing methods. FIG. 3(a) shows that the lossis balanced by depositing Au on the passive (un-modulated) arm tomaximize the ER; however, due to the ohmic loss nature of metal, the ILis higher compare to the second method. The dashed line indicates theoperating region for the present device, and additional 2.4 μm of Au canbe deposited on the active arm to increase ER whereas IL would alsoincrease simultaneously. FIG. 3(b) shows the first Y-branch hasarbitrary ratio to maximize ER, where the IL is also minimized since noextra loss induced on the passive arm. The splitting ratio denotes thepower distribution on the active arm in x percentage.

Returning to FIG. 1(a), the MZI output will be OFF when the light fromeach arm is out-of-phase (i.e., offset by 180 degrees), since the lightfrom each arm will cancel each other. And it will be ON when the lighton each arm is in-phase. The ITO 112 changes the index of refraction toadjust the phase of light and control the ON/OFF of the MZI underelectrical bias.

Thus, the inherent loss imbalance between the arms due to fabricationimperfections is a challenge in MZM schemes, which limits achievableextinction ratio (ER) defined as the ratio between maximum and minimumoutput power. It originates from the complex part of the optical phaseand leads to degraded optical signal fidelity (i.e. phase errors at theoutput and alter frequency chirps). Intrinsically, phase shiftingimpacts both the real and imaginary parts (i.e. K-K relations), thus theloss imbalance can alter during MZM operation between the ON/OFF states.The arm loss imbalance can be improved by tuning the MZM arm lossesstatically. However, a challenge in using an inherently lossy materialsuch as ITO, in an interferometric scheme similar to the MZconfiguration to achieve satisfactory modulation depth, is to match theamplitude of the optical signal (i.e. loss) in both arms of the MZ; theinterference from both arms at the output terminal converge referred tohereafter as balancing.

The passive MZI 100 is built on a silicon-on-insulator (SOI) substratewith the same waveguide lengths in both the arms, where subsequentprocess steps towards the active device include depositing ITO on top ofa portion of one arm separated by an oxide layer to facilitate gating,as shown in FIG. 1. A symmetrical passive MZI (i.e. same length for botharms and 50/50 Y-splitters on both sides) is chosen so that theinterference pattern at the output can distinguishably confer modulationeffects from our active ITO device. The MZM system can be simplified bycombining the field loss and phase shifts through each arm. The outputfield is then a simple function of the input field given by

E ^(%) _(out) =E ^(%) _(in)(a ₁ e ^(−iϕ) ¹ +a ₂ e ^(−iϕ) ² )  (1)

where a₁ and a₂ are the field gain (loss) in each arm of the MZI and ϕ₁and ϕ₂ are the phase induced through each of the arms (active/modulatedand un-modulated), respectively. The input and output fields are denotedby phasor quantities, i.e. E^(%)=Ee^(iωt), assuming there is no gain inthe system, (a₁+a₂)²≤1.

The time-dependent transfer function for the light intensity (or power)using the slowly-varying envelope approximation (modulationfrequency<<optical carrier frequency) is expressed as

$\begin{matrix}{T = {{\frac{E_{out}}{E_{in}}} = {a_{1}^{2} + a_{2}^{2} + {2a_{1}a_{2}\mspace{14mu} \cos \; \Delta \; \varphi}}}} & (2)\end{matrix}$

where

ϕ is the phase difference between the arms,

ϕ=ϕ₁−ϕ₂. To maximize the obtainable ER, i.e. ensuring minimal zeros inthe OFF state, the field losses in both arms need to be matched, i.e.a₁=a₂.

Deviations from this ideal case are typically attributed to imperfect50/50 Y-couplers. However, it is critical to emphasize that deviationfrom a₁=a₂ can be a result from differences in the losses anywhere inthe MZM configuration including possible fabrication imperfections. Incontrast, higher index change materials (e.g. ITO) do accompanysignificant loss as a byproduct of modulation and as such both thestates of operation need to be accounted for in design considerations.

The extinction ratio (ER) is the ratio of the transmission between theON (T_(max)) and OFF state (T_(min)), i.e. static ER since it ismeasured by varying a DC phase bias to one of the arms to find theabsolute maximum and minimum transmission. This is necessary since thedynamic ER may be reduced when operating at high frequencies due tolimited phase swings or pulse shaping from the finite bandwidth of theelectrodes. This upper bound can be referred to as the maximumextinction ratio, ER_(max). Defining γ=a₂/a₁ at as the field lossimbalance in the ON-state and, similarly γ′=a′₂/a₁ for the OFF state,where a′₂ denotes the OFF-state field loss since the change in fieldloss is not negligible for ITO resulting from the K-K relations, themaximum extinction ratio can be expressed as,

$\begin{matrix}{{ER}_{\max} = \left( \frac{1 + \gamma}{1 - \gamma^{\prime}} \right)^{2}} & (3)\end{matrix}$

Here, the field losses can be approximated as, a=e^(−αL) where L is thedevice length and the absorption due to the altered ITO material isα=2πκ_(eff)/λ; where λ is the operating wavelength, and κ_(eff) is theimaginary part of the effective index. With the aim to aim for ER_(max)and adjusting for both states of operation, we calculate the desiredlength of the metal contact, L_(m) on the un-modulated arm of the MZM,and chose to deposit metal (Au) on the other (un-modulated) arm of theMZM (FIG. 1) for two reasons: (a) it acts as our bottom electrode in themetal-oxide-semiconductor (MOS) stack (i.e. the Si waveguide is lightlydoped); and (b) imposes necessary loss on the un-modulated arm tofacilitate modulation depth, i.e. balancing the loss in both the arms.

When ITO is packaged as one electrode of an electrical capacitor,applying bias voltage can put the capacitor into the three known statesof accumulation, depletion, or inversion, thus changing the carrierconcentration. The optical property of the active material thereforechanges significantly, resulting in strong optical modulation. Inpraxis, a 1/e decay length of about 5 nm has been measured before, andmodulation effects have been experimentally verified over 1/e² (˜10 nm)thick films from the interface of the oxide and ITO. In order to extractrelevant parameters including the effective indices (real and imaginaryparts, n_(eff) and κ_(eff)) and confinement factors, Γ, we perform FEMeigenmode analysis for our structure (FIG. 1). The first ordertransverse magnetic (TM)-like mode is selected following theTM-optimized grating couplers in the fabricated device and the modeprofiles indicate an increase in the light confinement with modulationby 41% which is aligned with results from our previous work as weoperate away from the ENZ point in the n-dominant region (FIG. 1).

Since the modulation efficiency (ER/V_(pp)) is improved for betterelectrostatics, we use a relatively high-k dielectric, Al₂O₃, for thegate oxide of 10 nm using atomic layer deposition (ALD), followed by 10nm of ITO on the device region (ion beam deposition (IBD)). The latterhas synergies for processing ITO as this process yields densecrystalline films that are pinhole-free and highly uniform, and allowsfor a room temperature process, which does not anneal ITO (i.e. noactivation of Sn carriers as to facilitate electrostatic EO tuning).Incidentally, IBD technologies are advantageous for nanophotonic devicefabrication due to their precise controllability of material propertiessuch as microstructure, non-stoichiometry, morphology, crystallinity,etc.

Results show a modulation depth (i.e. ER) of ˜2.1 dB for a phase shifterlength of only 32 μm (FIG. 2). The voltage needed for π-phase shifts atthe output is about 16 V (FIG. 2) gives a corresponding V_(π)L of just0.52 V·mm. The output intensity of the MZ configuration is governed by

$\begin{matrix}{I \propto {E}^{2} \propto {\cos^{2}\left( \frac{{\Delta\beta}\; L}{2} \right)}} & (4)\end{matrix}$

where, the output intensity is normalized such that the peaktransmission factor is 1 for ideal power transmission,

β is the induced change in the propagation constant, β between both armsduring modulation. I-V measurements of the device exhibit activecapacitor operation in the reported range away from capacitor saturationregion or gate oxide breakdown (FIG. 2).

The small non-zero current at 0 V corresponds to the capacitor chargingeffects from the continuous bias sweeping measurement. The change in theeffective index of the waveguide, Δn_(eff)≈0.020, for ON/OFF modulationis estimated using the applied voltage, V_(d) and the material indexchange in the ITO, Δn_(ITO) from FIG. 2, which closely matches the valueobtained from the FEM analysis (˜0.023). The effective index change withmodulation can be found by a linear approximation with applied voltageas ∂n_(eff)/∂V_(d)˜1.407×10⁻³ V⁻¹. Both the effective index change,Δn_(eff) and material index change in ITO, Δn_(ITO) exhibit monotonicincrease with applied bias in our experimental voltage range. This isexpected as we operate in the n-dominant region of the ITO material farfrom any ENZ effects. Note, the change in both the indices correspond toa decrease in the corresponding indices as modulation assimilates toblue-shifts in device resonance, however is hardly resolvable in oursingle pass MZ configuration (FIG. 2), but is well-known from ringresonators, photonic crystal cavities or any Fabry-Perot cavity.

The weak dispersion of ∂n_(eff)/∂Vλ˜1.12×10⁻⁴ nm⁻¹ in our modalstructure contributes to the undistinguishable resonance shift in thetransmission spectra. The modal dispersion is calculated from Eigenmodeanalysis. The material index change in ITO, Δn_(ITO) shows near unityorder index change as demonstrated for this emerging material (FIG. 2),which is significantly higher as compared to its Si counterpart whileboth (ITO and Si) operate with the free carrier modulation mechanism.This improvement of ITO can be attributed to: (a) 2-3 orders highercarrier density, and (b) the higher bandgap, which consequently leads toa lower refractive index. If the change of the carrier concentrationδN_(c) (e.g. due to an applied voltage bias) causes a change in therelative permittivity (dielectric constant) δε, the corresponding changein the refractive index can be written as δn=δε^(1/2)˜δε/2ε^(1/2);hence, the refractive index change is greatly enhanced when thepermittivity, s is small. The noticeable change from the monotoniceffective indices trend originates from modal confinement increasingwith bias, i.e. as we tune towards the ENZ region the confinementincreases without actually biasing it to the ENZ region.

The simple biasing scheme used here to repurpose the Si waveguide as abottom contact in the MOS-stack, however, severely limits modulationspeed of this device due to high electrical resistance; the epi-Si layerof the SOI substrate is only lightly doped thus R_(Si)˜600 MΩ. Thecontact and sheet resistance of the ITO film is ˜220Ω and 63Ω/□,respectively. The resistivity and mobility of the ITO film is measuredto be 6.36×10⁻⁴ Ω-cm and 42.6 cm²/V-s, respectively. The devicecapacitance is ˜170 fF and hence projected speed is only ˜1.5 kHz.Because of the design decision to employ the Si-contact on theun-modulated arm, the device is resistance (R)-limited, which could befurther optimized by Si selective doping (about 2 orders of magnitude)and bringing the Si contact closer to the active region.

Selective plasma treatments on the ITO contact region can avail lowercontact resistances up to another 2 orders of magnitude. The switchingspeed of such modulators are essentially limited by the dynamics ofmajority carriers in the ITO film and optimally speeds in GHz rangesshould be feasible as demonstrated in other majority carrier baseddevices. Hall effect measurements revealed the carrier concentration ofthe as deposited ITO film, N_(c)=2.29×10²⁰ cm⁻³. The change in thecarrier concentration level arising from active capacitive gating iscalculated as ΔN_(c)=1.1×10²⁰ cm⁻³ utilizing both accumulation anddepletion mechanisms depending on applied bias considering the 10 nmblock of the ITO material.

In addition, an analysis of ITO's modulated carrier concentrationprofile can be implemented using the modified Thomas-Fermi approximation(MTFA) method characterized by the modified Thomas-Fermi approximatescreening length, λ_(MTFA)˜3 nm. MTFA allows for the quantum-mechanicalinfluence of an infinite potential barrier at the surface and has showncompliance for semiconductors with surface band bending at thenanoscale. Even with MTFA, the carrier density change in our ITOaccumulation layer is ΔN_(c)=3.6×10²⁰ cm⁻³, resulting in carrierconcentration levels from 4.9×10¹⁹ cm⁻³ to 4.1×10²⁰ cm⁻³ correspondingto the capacitive gating which is still away from reported ENZ region of6˜7×10²⁰ cm⁻³. The dynamic switching energy is about 11 pJ/bit usingcapacitive charging.

The limited ER found indicates that the loss balancing in both of thearms are imperfect, which can be attributed as combinations of severalfactors such as the passive waveguides being non-identical (sidewalls,roughness, etc.), inadequate Y-splitters skewed astray from 50:50 ratio,fabrication conditions and imperfections, dissimilarity between the asdeposited materials (e.g. ITO, Au, Al₂O₃) from used values in the FEManalysis or analytical expressions in design, film quality,non-uniformity of the oxide or metals, etc. The loss imbalance can beestimated form the visibility of the interferometric output. Forimbalanced lossy MZ schemes the visibility can be written as

$\begin{matrix}{v = \frac{1}{\cosh \left( {{\Delta\alpha}_{bal}L_{bal}} \right)}} & (5)\end{matrix}$

where, Δα_(bal) is the amount of loss (absorption) required to bring thesystem to balance and hence improve the ER, and L_(bal) is thecorresponding length needed of the imposed lossy material. We calculatethe visibility of the fringes from our results as v=0.228, which leadsto an imbalance factor Δα_(bal)L_(bal) of 2.16. This translates to ˜2.4μm of additional Au placed on the active arm to balance the loss andimprove ER (FIG. 3(a)). However, this would further limit the outputpower by enhancing the insertion loss (IL) possibly refraining furtherdetectable measurement.

Mechanisms to enhance device performances can include designing theinput Y-splitter with a power splitting ratio to compensate the lossresulting as a byproduct of active modulation (FIG. 3(b)); i.e. evaluatean input Y-splitter with arbitrary splitting ratio, x in terms of thepower flow into the active arm. Results show that an input Y-splitterwith ˜75% power flowing into the active arm can compensate for themodulation loss arising from K-K relations towards maximizing ER.Furthermore, this approach is beneficial in terms of the IL as it doesnot have to encounter any parasitic metallic loss as in the presentdesign. Biasing the MOS-stack can propose a challenge in thisconfiguration which can be addressed by a small protrusion of the Siwaveguide underneath the active region with selective doping treatmentsto form a contact pad. Back reflection caused by such protrusion needsto be taken into account also which is not focused on here and canstimulate future research.

An increase in loss (i.e. decrease in output power) of about 6 dB fromthe passive to active device was found which is an upper bound IL forthe active device. However, the actual IL of the device might be loweras this increase of loss refers to the change in loss from theunprocessed passive MZ structure to the active device after allprocessing and subsequent processing on the same chip is known tointroduce additional loss with every process step (eg. patterning,liftoff etc.). The estimated efficiency-loss product of the device,αV_(π)L is about 80 dB·V at the ON state. This value is rather high forconventional low loss and low index variable active materials (e.g. Si,III-V, etc.), but comparable with emerging highly tunable material baseddevices such as graphene. This high value also reflects the loss balanceimperfections; balanced schemes as shown in FIG. 3 could help tooptimize this value down to comparable limits with conventional Si basedschemes (˜20 dB·V).

The present invention demonstrates the first ITO-based Mach Zehndermodulator on a Silicon photonics platform which can enable compactmodulator footprint, ease of fabrication and attain CMOS compatibility.Our results confirm a near unity order index change in the ITO materialupon applied bias and obtained a low V_(π)L of 0.52 V·mm. Although thespeed limitation in this work does not warrant data communication, suchresponse rates (˜ms) can avail applications in phased array systemswhich can be beneficial in emerging technology such as light detectionand ranging (LiDAR) for terrestrial and areal localization and mapping.This invention provides pathways for future optimization schemesfacilitating improved device performances including high-speedoperation, enhanced modulation depth, and reduced insertion loss. toavail dense on-chip integration for data communication.

Plasmonic ITO Lateral Capacitor MZI

FIG. 4(a) is a schematic representation of a lateral MOS ITO based MachZehnder Interferometer 140 operating at λ=1550 nm. Here, the phaseshifter length (between the contacts) is <2 μm. As shown, the MZI 140 issplit to form a first arm 141 with a passive contact, and a second arm143 with an active module. The active module includes a phase modulator142, a biasing metal contact 144 on the phase modulator active material142, a gate dielectric (oxide, Al₂O₃) 147, and a ground contact 150 forcompleting the lateral capacitor. The phase modulator can be anysuitable element that modulates the phase of light traveling through thearm waveguide. In the current example embodiment, the phase modulator isan optical index modulator that changes the index of refraction of lighttraveling through the arm waveguide with functionally biasing the activematerial in this lateral capacitive configuration, and is specificallyshown as ITO being the active modulation material.

The ITO 142 contacts and covers or surrounds at least a portion of thesecond arm 143, and the metal contact to the ITO 144 contacts and coversor surrounds at least a portion of the ITO 142. The ground contact 150contacts and covers or surrounds at least a portion of the second arm143 and is set apart from the ITO 142. The biasing contact 144 andground contact 150 can be gold. The oxide layer 147 is partially etchednear the active device region (right side of 142) to facilitateelectrostatic field overlap from the active contacts 144 and 150interacting favorably with the active material 142.

Thus, in FIG. 4(a), the ground 150 is on the same arm as the ITO metalcontact 144, which provides a desired field shown in FIG. 5(c) and FIG.5(d); whereas in FIG. 1(a) the ground is on a separate arm than the ITOand contacting the Si waveguide, which makes the capacitor configurationformed by the Si and ITO electrodes in FIG. 1(a) Si/oxide/ITO with aphotonic mode whereas in FIG. 4(a) the capacitive stack is plasmonicmetal/oxide/ITO laterally, and the capacitor is formed by the ITO andmetal (ground) electrodes (FIG. 4(a)). A drive or bias voltage V_(bias)is applied to the metal ITO contact 144, which transfers to the ITO 142,as shown. As further shown, the ITO 142, ITO contact 144, oxide layer147, and ground contact 150 can be formed from thin layers.

FIG. 4(b) is an optical microscope image of the fabricated Mach Zehndermodulator with contact pads for biasing shown. The patterned ITO 145 andthe partial oxide etched region 146 are highlighted with dashes. FIG.4(c) shows schematic diagrams of the device in: (i) Cross-sectional viewin the active ITO deposited region; and (ii) A longitudinalcross-section along the Si waveguide exhibiting the device region withthe partial etched gate oxide region. The Al₂O₃ gate oxide is 10 nmthick, and partially etched down to 1-2 nm thickness in white dashedarea in FIG. 4(b). Si waveguides are 500 nm×220 nm, deposited ITO layeris 10 nm thick.

To maximize the obtainable ER, i.e., ensuring minimal zeros in the OFFstate, the field losses in both arms need to be matched, i.e., a₁=a₂.Deviations from this ideal case are typically attributed to imperfect50:50 Y-couplers. However, it is critical to emphasize that deviationfrom a₁=a₂ can be a direct result from differences in the lossesanywhere in the MZM configuration including possible fabricationimperfections. By contrast, higher index changeable materials (e.g.,ITO) do accompany loss as a byproduct of modulation and as such both thestates of operation need to be accounted for in design considerations.One can improve this arm loss imbalance by tuning the un-modulated armlosses statically to counteract the imbalances arising from the K-Krelations. As such, we chose to deposit metal (Au) on the other(un-modulated) arm of the MZ (FIG. 4(a)) 141.

Since the modulation efficiency (ER/V_(pp)) is improved for betterelectrostatics, we use a relatively high-k dielectric, a 10 nm oxidelayer of Al₂O₃ is grown on the passive structure using atomic layerdeposition (ALD) to aid capacitive gating schemes. Subsequently, a 10 nmthin film of ITO is deposited using an ion beam deposition (IBD) processafter necessary patterning using EBL and liftoff processes afterwards(FIG. 4(b), gray area underneath the right contact pad). The IBD processhas synergies for processing ITO as this process yields densecrystalline films that are pinhole-free and highly uniform and allowsfor a room temperature process, which does not anneal ITO (i.e., noactivation of Sn carriers as to facilitate electrostatic EO tuning). IBDtechnologies are advantageous for nanophotonic device fabrication due totheir precise controllability of material properties such asmicrostructure, non-stoichiometry, morphology, and crystallinity.

A selective etch step of the ALD grown oxide near the active ITO deviceregion is enacted to facilitate the electric field overlap from thecontacts with the active ITO material 146 (FIG. 4(b)). Contacts and theplasmonic top layer are formed by depositing 50 nm of Au using electronbeam evaporation process. An adhesion layer of 3 nm of Ti is used in theprocess. The other contact is placed in close proximity (<2 μm) to theplasmonic top contact in the partial etched region to maximize theelectrostatic field overlap to the active ITO region (FIG. 4(c)(i)). Theschematic of a longitudinal cross-section along the Si waveguide (activearm of the MZI structure) in the device region is illustrated in FIG.4(c)(i) and a cross-sectional schematic of the active plasmonic ITOregion is shown in FIG. 4(c)(ii). Another contact on the partial etchedregion is placed for determining the partial etch success, as we aimedfor a remainder of just 1-2 nm thin oxide film after etch (FIG. 4(b)),this contact provided the control to determine if etched all the waythrough to the conductive Si layer.

The pattern transfers were performed in EBL using the Raith VOYAGER toolwith PMMA based photoresists, and MIBK:IPA (1:3) developer for 60 s. 50nm of Au for contacts and the plasmonic top layer in the mode structurewere deposited using an e-beam evaporation system (CHA Criterion) as Auhas reasonably low ohmic loss at near IR wavelengths. An additional 3 nmadhesion layer of Ti was used in the contacts. The Al₂O₃ oxide wasdeposited using the ALD technique as it provides reliable and repeatableperformance characteristics. The Fiji G2 ALD tool was used at lowtemperature settings (100° C.) for 100 cycles to deposit about 10 nm ofAl₂O₃ to ensure higher film quality devoid of any pinholes or surfacetraps. A Filmetrics F20-UV system was used to characterize the Al₂O₃deposition rate.

An etch step was required for the partial etch near the active deviceregion to facilitate the necessary height contrast between the plasmonictop contact and the lateral bottom contact in the MOS-stack. We used arather slow wet etch process for Al₂O₃ using an MF319 solution in thearea of interest (near the active ITO region, keeping the both contactssufficiently close in proximity without jeopardizing etching on theextended active ITO region). Note, MF319 contains tetramethylammoniumhydroxide (TMAH), which reacts with the Al and can etch the oxidethereof.

Experimental I-V measurements of the device show a working capacitor inthe measured voltage range, not showing any observable saturation of theMOS capacitor or breakdown of the gate oxide characteristics (FIG.5(a)). Electro-optic transmission power tests via a gated-transmissionmeasurement exhibit reasonable modulation of the laser powerdemonstrating a modulation depth (i.e. ER) of ˜1.34 dB in the measuredbias range, and a squared cosine fit (as dictated by the underlyingphysics of MZIs from Eq. (4)) can obtain an ER of 2.2 dB. The quality ofthe fit symbolized by the coefficient of determination (R²) is 0.86. Thevoltage needed for π-phase shifts at the optical output is about 33 V(FIG. 5(b)) and a corresponding V_(π)L of just 63 V·μm given the <2μm-short phase shifter.

Thus, FIG. 5(b) shows normalized output optical power, P_(out) (a. u.)vs. bias voltage, V_(bias) (Volts) demonstrating the modulatorperformance, the experimental data was fit with a squared sinusoidal(cos²(arg)) fit to extract the half-wave voltage, Vπ≈33 V. FIG. 5(a)shows I-V measurements of the ITO-oxide-metal lateral capacitive stack;a Savitzky-Golay smoothing function was applied on the experimental datato showcase the I-V characteristics. FIG. 5(c) shows FEM simulation ofthe longitudinal cross-sectional device region revealing electrostaticfield overlap with the active ITO in the device region arising frombias. FIG. 5(d) shows detail of the field line interaction with the ITOlayer. The line plot represents the carrier concentration, which is lowat ground and high at (above and below) the ITO.

In order to gain more insights into the field-distribution of thislateral-gated modulator, finite element method (FEM) simulations arecarried out to resolve the electrostatic field overlap with the activeITO arising from capacitive gating which confirms an increased fieldoverlap due to the partial etching of the oxide (FIGS. 5(c), 5(d)). Herewe use the aforementioned carrier concentration of 2×10²⁰ cm⁻³ and1×10¹⁴ cm⁻³ for ITO and Si, respectively. The plasmonic opticalconfinement in the active region further acts to amplify the materialindex change into obtainable effective modal index variation; henceaiding the overall modulation depth arising from both effects(traditional plasmonics and improved electrostatics in the lateralconfiguration). As both plasmonic top contact and the bottom contact inthe capacitive stack are only metal paths, there is little resistanceleading up to the device region; so, such a device is only limited bythe capacitance (not R-limited) in terms of attainable speed. Selectiveplasma treatment on the ITO contact region can avail lower contactresistances up to 2 orders of magnitude. The switching speed of suchmodulators are essentially limited by the dynamics of majority carriersin the ITO film, and optimally, speeds in GHz ranges should be feasibleas demonstrated in other majority carrier-based devices.

Plasmonic ITO MZI Vertical Capacitor

The plasmonic design for a vertically configured capacitor is shown inFIG. 6 and provides commensurate performance in terms of the FOM. Thelow contact resistance can help bring down the R-limited threshold forthe photonic structure in the ITO/oxide/ITO and metal/oxide/ITOstructures; essentially enabling high speed operation. FIG. 6(a) is aschematic representation of cross-sectional structures for both arms ofan MZI; the top for the passive contact region for balancing the loss inboth waveguide outputs and the bottom for the active capacitive region.FIG. 6(b) is a plasmonic vertical MOS ITO based Mach ZehnderInterferometer 164 operating at λ=1550 nm. Here, the phase shifterlength is the actual device 165 length.

As shown, the MZI 164 is split to form a first arm 155 with a passivecontact 156, and a second arm 158 with an active module. The activemodule includes a phase modulator 165, a biasing metal contact 162 onthe phase modulator active material 159, a gate dielectric (oxide,Al₂O₃) 160, and a ground metal contact 157 for completing the verticalcapacitor. The phase modulator can be any suitable element thatmodulates the phase of light traveling through the arm waveguide.

In the current example embodiment, the phase modulator is an opticalindex modulator that changes the index of refraction of light travelingthrough the arm waveguide with functionally biasing the active materialin this vertical capacitive configuration with the ITO 159 as oneelectrode and the metal 161 as the second electrode separated verticallyby an oxide layer 160, and is specifically shown as ITO being the activemodulation material. The ITO 159 contacts and covers or surrounds atleast a portion of the second arm 158, and the metal contact to the ITO162 contacts and covers or surrounds at least a portion of the ITO 159away from the waveguide 158 (FIG. 6(c)). The ground contact 157 contactsand covers or surrounds at least a portion of the second arm 158 and isset on top the ITO 159 separated by a gate oxide, Al₂O₃ 160. The biasingcontact 162 and ground contact 157 can be gold. The TCO material beingconductive forms a vertical capacitor stack (i.e. the phase shifter 165here) on top of the waveguide 158.

Thus, in FIG. 6(c), the ground 157 is on the same arm as the ITO metalcontact 162, which provides a desired vertical capacitance to induce thephase change in the waveguide 158; whereas in FIG. 1(a) the ground is ona separate arm than the ITO are contacting the Si waveguide, which makesthe capacitor configuration in FIG. 1(a) Si/oxide/ITO with a photonicmode, and in FIG. 4(a) the capacitive stack is plasmonic metal/oxide/ITOlaterally; whereas here in FIG. 6 the capacitor and optical modecombination is plasmonic and in vertical metal/oxide/ITO formation. Adrive or bias voltage V_(bias) is applied to the metal ITO contact 162,which transfers to the ITO 159. As further shown, the ITO 159, ITOcontact 162, oxide layer 160, and ground contact 157 can be formed fromthin layers.

FIG. 6(d) is an optical microscope image of the fabricated Mach Zehndermodulator with contact pads for biasing shown. The patterned ITO 159 andthe ITO contact 162 and the plasmonic top metal 161 and the metalcontact 157 are shown (FIG. 6(d)). Si waveguides are 500 nm×220 nm,deposited ITO layer is 10 nm thick and the Al₂O₃ oxide layer is 20 nmthick. FIG. 6(e) shows the modulation performance indicated by thevarying normalized optical output with bias voltage at almost 3 dB. Asquared sinusoidal fit, as dictated by the underlying mechanism of MZIsin Eq. (4), is employed to ascertain the half-wave voltage of ˜38.3 V.The corresponding FOM, V_(π)L is miniscule at 95 V·μm showcasing theeffective modulation in this vertical MOS-capacitor plasmonicconfiguration with a compact device length of just 2.5 μm. FIG. 6(e)also depicts the I-V characteristics of this vertical capacitor in themeasured voltage range assuring working capacitor traits away from anysaturation or breakdown regions. FIG. 6(f) shows FEM eigenmodes of theactive device cross-section for TE and TM modal polarizations in thevertical capacitive plasmonic structure corresponding to the ON and OFFstates of operation.

1-D ITO PCNB

Turning to FIG. 7, a vertical Silicon slot waveguide is shown, with athin ITO film provided in the dielectric slot gap region. FIG. 7 is aschematic of the 1-D vertical slot ITO PCNB cavity with air holes, inwhich FIG. 7(a) is a 3D perspective view, FIG. 7(b) is a top view, andFIG. 7(c) is a side view (longitudinal cross-section) showing the mirrorsection and tapered section holes forming the Fabry-Perot-like cavity.As shown, the waveguide is formed with a top Si layer, then an HfO₂layer, then an ITO layer, then an HfO₂ layer, then a bottom Si layerconstituting the vertical Si slot.

The air holes can extend the entire width through the waveguide, fromthe top to the bottom, and through all of the intermediary layers. Theair holes can all be the same size and shape and spaced evenly along thelength of the waveguide, or can be different sizes and/or shapes and/ordistances from each other. Thus, as shown, the waveguide can have endsections with holes of equal size, then a middle tapered section withholes of gradually smaller size. In one embodiment, the cavity length,L_(cavity)=100 nm, hole period of mirror section, a=365 nm; minimum holedistance of taper section, a_(min_taper)=230 nm; hole radius, r=0.28a;number of taper hole pairs, n=14; number of mirror hole pairs, m=18;waveguide height, h=320 nm. The cross-sectional mode structure with theITO thin film sandwiched between two p-Si claddings with oxides oneither side for electrostatic gating are shown magnified as an inset toFIG. 7(c). The relevant parameters are: W=300 nm, h_(Si)=150 nm,d_(ox)=5 nm, and d_(ITO)=10 nm.

This modal configuration allows the slot waveguide mode to be tightlyconfined to the active (ITO) region. In fact, suchsemiconductor-insulator-semiconductor (SIS) mode allows sub-diffractionlimited optical modes despite the absence of metals. Here, the jump ofthe dielectric permittivity across the high index silicon claddings andthe low index center section (comprised of oxide/ITO/oxide) allows foroptical confinement in the SIS configuration (FIG. 7). With a certainbias voltage, both the propagation constant and absorption coefficientof the fundamental transverse magnetic (TM)-like mode changeconsiderably. The SIS slot waveguide uses a 10 nm ITO thin film layersandwiched between two p-Si layers of 150 nm separated by two oxidelayers each 5 nm to facilitate gating (FIG. 7(c)).

This ITO layer can be considered as a degenerate semiconductor with alarge number of electrons. For the oxides, we selected the high-kdielectric HfO2 for the following two reasons: a) HfO₂ has a high staticpermittivity (∈_(HfO) ₂ ˜25) which increases the electrical capacitance,and thus allows lowering the gating voltage in tuning this cavity, thushelping to lower the energy overhead of active reconfiguration; and b)HfO₂ thin films can be reliably grown with the atomic layer deposition(ALD) process. A 1D photonic crystal nanobeam cavity (PCNB) incorporatedwith the ITO vertical slot waveguide is formed by creating verticalopenings forming the mirror and taper sections. The nanobeam cavityconsidered here is essentially a wavelength-scale Fabry-Pérot etalonformed by sandwiching a defect section between ID photonic crystal Blochmirrors and tapered sections.

Light in the longitudinal direction in the nanobeam is confined by aprinciple of total internal reflection (FIG. 7(a)). Similar to aFabry-Perot spacer, the mirror and tapered section holes at the centerof the PhC effectively confine the light in the propagating directioninside the central cavity section by the principle of total internalreflection from the mirrors formed by the air holes. In other words, thetaper sections achieved using a 14-hole linear taper are only used forimproving mode impedance mismatch and minimizing the reflection. Thedesign parameters of the 1D PCNB cavity include the cavity length of 100nm; hole period of mirror section, a is 365 nm; minimum hole distance oftaper section, a_(min_taper) is 230 nm; hole radius, r=0.28a; number oftaper hole pairs, n=14; number of mirror hole pairs, m=18; waveguideheight, h is 320 nm. This ID photonic crystal is simulated by theexcitation of a fundamental TM-like 1^(st) order dipole source from thez-axis and the length span of the device is in the x-direction. Thelength and width of the PCNB structure are 23 μm and 300 nm,respectively (FIGS. 7(b), 7(d)).

The PCNB structure is integrated into a typical SOI substrate with aburied oxide of 1 μm thickness. The high confinement of the mode in theactive region in the ITO away from the substrate, with symmetric Siupper and lower claddings, allows insignificant leakage of light intothe substrate. Conceptually this cavity could be fabricated in eithertop-down or bottom-up approaches. The latter may, however, bechallenging since deposition conformity across varying mechanismsrequired for the various layers, may not result in a clean lift off. Incontrast, in a top-down approach one can form the proposed cavitystructure depositing all layers followed by one single patterning stepand etching the air holes on the vertical slot waveguide using a hardmask (e.g. Cr). The layered materials (Si, HfO₂ and ITO) can be etchedby reactive ion etching, for instance, using CF₄ and subsequently Cretchants (e.g. CR7) can be used to etch away the hard mask. The carrierconcentration levels, effective indices and extinction coefficientscorresponding to the states of operation in this work are listed inTable 3.

TABLE 3 ITO carrier concentration levels, effective indices andextinction coefficients corresponding to the states of operation for thetunable PCNB cavity Modal effective index, ñ_(eff) ITO Carrier RealImaginary States of operation Concentration (cm⁻³) part, n_(eff) part,k_(eff) ON 10¹⁹ 2.27 2.14 × 10⁻⁴ OFF 10²⁰ 2.25 2.53 × 10⁻³

FIG. 7(d) shows the electric field intensity, |E|², distributionprofiles of the cavity mode. TM-like 1st order mode of 1D PCNB cavityshowcased by |E_(XY)|² is recorded in the xy-plane at the cavity centeralong the z-direction. The cavity mode is excited by placing an electricdipole source with z-orientation inside the vertical slot cavity. The1-D ITO PCNB cavity exhibits a TM-like fundamental 1st order mode (i.e.,dominant E-field in the z-direction) (FIG. 7(d), right side). Theelectric field of the TM mode extends longitudinally along thewaveguide, which is consistent with free-standing dual-polarized siliconPhC nanobeam cavities. In addition, the electric field profile isconfined in the low index active region between these air holes. Asexpected, light-matter interaction (LMI) is enhanced in this type ofstructure compared to a bulk photonic structure due to increasedconfinement. The feedback from the mirror sections and subsequentimpedance matching from the tapered sections allow the confinement oflight inside the cavity (centered at x=0) in the longitudinalpropagating direction (x-orientation), which is evident in the cavityprofiles for the xy and yz-plane (FIG. 7(d)). The cavity as a means ofstorage for the energy (i.e. |E|²) is also discernible from the cavityxy and xz profiles as both exhibit the highest field intensity near x=0(cavity region).

Referring to FIG. 7(e), transmission intensity versus wavelength,corresponding to states of operation is shown. The tunability of thecavity is shown in the inset near the telecom O-band, the change inwavelength ˜3.4 nm. In FIG. 7(f), the quality factor, Q and Purcellfactor, F_(p) of the ITO vertical slot PCNB cavity is compared to asimilar Si₃N₄ ID PCNB cavity.

Tuning is an effect of change in the effective refractive index, n_(eff)The subsequent broadening of the cavity resonances with respect towavelength can be related back to the loss in the modal absorption,corresponding to the effective extinction coefficient, κ_(eff). Theshift in resonance with the change in carrier concentration, i.e.tuning, Δλ, results from the change in the effective refractive index(real part), Δn_(eff), from the aforementioned modal tuning properties.Our results show a considerable amount of wavelength tuning, Δλ˜3.4 nm,for the slot ITO PCNB cavity (FIG. 7(e)). From ON to OFF state, thecavity exhibits blue-shifts in the resonances originating from theeffective index decrease. Loss also increases with tuning as a directresult from the Kramers-Kronig relations.

Regarding the tuning range (i.e., carrier concentration levels), we havemanaged to keep this and subsequent broadening of the resonances low dueto: a) selectively reducing the allowable carrier concentration tuningrange in the n-dominant region away from the ENZ region, and b) byselecting a low carrier concentration initial point (ON state) of 10¹⁹cm⁻³, where ITO behaves as a dielectric. As the resonances broaden withtuning, the cavity Q-factor is also reduced. This subsequent losstrivially lowers the finesse (F) of the cavity, hence only slightchanges in the Q-factor are introduced despite the higher effectiveindex (κ_(eff)) change, since increase in mirror quality with tuning(higher reflection since more metallic) is compensated by the higherloss.

For the ITO slot PCNB cavity we achieve the Q value of ˜774 (695) in theON (OFF) state, where the lower Q-value of the OFF state originates fromthe modal absorption increase (FIG. 7(h)). The Q-factors calculated hereuse a low-Q cavity method used in the Lumerical FDTD solutions, which isdetermined through the Fourier transform of the field by finding theresonant frequencies (f_(R)) of the signal and measuring the full widthhalf maximum (FWHM, Δf) of the resonant peaks, i.e., Q=f_(R)/Δf, if theelectromagnetic fields decay completely from the simulation in a timethat can be reasonably simulated by FDTD.

The longitudinal (z-axis) confinement leads to the necessary cavityfeedback to facilitate stimulated emission (lasing), if optically orelectrically pumping the gain medium. In recent history, the Purcellfactor, F_(p), is used to describe the LMI in laser physics as itrelates to a measure of the spontaneous emission rate enhancement of adipole emitter source placed in the cavity compared to internalradiative recombination rates in a homogeneous semiconductor material,given by

$\begin{matrix}{F_{p} = {\frac{3}{4\pi^{2}}\left( \frac{\lambda_{R}}{n} \right)^{3}\left( \frac{Q}{V_{mode}} \right)}} & (6)\end{matrix}$

where λ_(R) is the resonant free-space wavelength of the cavity, in isthe real part of the complex refractive index at the field antinode, andV_(mode) can be estimated from a commonly used definition as

$\begin{matrix}{V_{mode} = \frac{\int{\in {{{E(r)}}^{2}{dV}}}}{\max \left\{ {\in {{E(r)}}^{2}} \right\}}} & (7)\end{matrix}$

where ∈ is the dielectric constant, E(r) is the electric field strength,and V is a quantization volume encompassing the resonator and with aboundary in the radiation zone of the cavity. Eq. (6) indicates that alarge Q and a smaller V_(mode) are desired to enhance emission rate(i.e. F_(p)). Evidently, there exist two techniques to increase thePurcell factor—the classical approach is enhancing the cavity Q-factor,and the other one is minimizing the mode volume, V_(mode).

However, the former is rather unpractical since it not only requiresincreased wafer space, but also has negative effects for datacommunication applications, particularly for lasers driven in a directmodulation mode due to the long cavity photon lifetimes inside thehigh-Q cavity, τ_(ph)=Qλ/2πc, where λ is the operating resonantwavelength and c is the speed of light in vacuum. Although the smallmode volumes of PCNB cavities can be attained by a proper design, high-Qfactors are typically obtained using extensive parameter search andoptimization.

As our slot structure can squeeze in the light in the active region dueto higher confinement arising from the index contrast, the small modalarea leads to a miniscule diffraction limited mode volume in a cubichalf-wavelength in material of ˜0.1 (λ/2n)³ in the cavity. Since Q isultimately limited in practice by other factors, such as bandwidthconsiderations, material absorption, or fabrication tolerances,minimizing V_(mode) for a given Q is a preferred solution with practicalapplications in mind. Such optical cavities enable spontaneous emissionrates that are faster than the stimulated emission rates for nanoscalelight-emitting devices.

Note, this is not only limited to nanoscale-based laser cavities butalso can be applied to conventional laser designs as well. On the otherhand, the internal dynamics leading toward the laser threshold are moreefficiently utilized when a smaller optical mode volume is used (i.e.,higher F_(p) and spontaneous emission coupling factor, β). The case witha smaller mode volume often translates into a low power requirement. Itis worthy to mention that while traditional techniques involving tunablelasers utilize changing the feedback system of the cavity usually bychanging the cavity length (i.e. effective path length), our proposeddesign can utilize the tunability from effective index tuning in thecavity free from moving parts and providing degrees of freedom indesign.

Comparing the cavity performance of a Si₃N₄ ID PCNB cavity from Liu K.and Sorger V. J., 2015 (Enhanced interaction strength for square plasmonresonator embedded in a photonic crystal nanobeam cavity, J.Nanophotonics 9093790) to our results shows that the Q-factor hasdeclined by about one order of magnitude because of the addition of thelossy ITO, yet the Purcell factor has risen by about the same amount(FIG. 7(f)); in detail, Q-factor ˜10,000 for the Si₃N₄ PCNB to ˜770 forthe ITO slot PCNB, which can be contributed to the inherent orders ofmagnitude higher loss in the ITO material. However, we observe asignificantly higher Purcell factor enhancement from ˜620 for the Si₃N₄PCNB to ˜4820 (˜4320) for the ITO slot PCNB cavity ON (OFF) state. Thiscan be attributed to the strong modal confinement in our vertical slotstructure, essentially squeezing the light into the active regionarising from sufficient index contrast in the mode. As such, we are ableto reduce the effective modal area resulting in a modest small modevolume.

Graphene/Oxide/ITO EAM

An absorption modulator with graphene 173/oxide 172/ITO 171 capacitivestack on top of a linear Si waveguide 170 embodies the current inventionas shown in FIG. 8(a), whereas any TCO/dielectric/2-dimensional materialcan be used in similar manner. The area of the ITO film in contact withthe waveguide is defined through electron beam lithography (FIG. 8(b)).Afterwards, the 10 nm film of ITO 174 is deposited on top of one of thepassive waveguides 170 on SOI substrate at room temperature via ion beamdeposition (IBD) using the 4Wave IBD/BTD cluster sputter depositionsystem. An RF ion gun focuses Ar ions onto substrate targets of ITO. TheITO target stoichiometry is 90 wt. % In₂O₃/10 wt. % SnO₂. A small flowof O₂ (2 SCCM) is used. The process used an Ar flow rate of 16 SCCM, abeam voltage of 600 V, a beam current of 220 mA, and an accelerationvoltage of 150 V. The sample was set at an angle of 115° and rotated at10 rpm to ensure smooth profile. The deposition rate used was 0.77 Å/s.The temperature for the process was 20° C. to refrain annealing effects.The base vacuum used was 2×10⁻⁸ Torr, and the deposition uniformity wasconfirmed as 1.5% (1σ) over a 190 mm diameter.

The Al₂O₃ oxide 172 was deposited using the atomic layer deposition(ALD) technique as it provides reliable performance characteristics. TheFiji G2 ALD tool was used at low temperature settings (100° C.) for 200cycles to deposit about 20 nm of Al₂O₃ to ensure higher film quality andto avoid any annealing effects to the ITO. An etch step was required ontop of the ITO contact 174 to remove the oxide over it for electricalprobing. We used a rather slow wet etch process for Al₂O₃ etching usingan MF319 solution in the contact pad area. MF319 containstetramethylammonium hydroxide (TMAH) which reacts with the Al and canetch the oxide thereof. Next, monolayer graphene film 173 (Graphenea,Easy Transfer) was wet transferred onto the substrate and then patternedby EBL (Raith Voyager) with negative photoresist (AR-N 7520) followed byoxygen plasma etching. After graphene patterning, an EBL step definedthe contact pads 175, followed by Ti/Au deposition and lift-off process.The optical image of the device in FIG. 8(b) depicts anelectro-absorption modulator, with a lateral side of just 10 μm.

Results highlighted in FIG. 8(c) show a 1.5 dB extinction ratio when a 5V bias voltage is applied (V_(bias)) between the ITO layer and graphenein a capacitor configuration. The modulator has a higher modulationrange (higher absorption) compared to a hypothetical device constitutedby either a single layer graphene or single layer ITO or dual layergraphene, since both ITO and graphene are concurrently contributing inabsorbing light when electrostatically tuned and the mode overlap withITO is quite significant. This device configuration, thanks to the lowRC delay enables high speed (up to few tens of GHz). Furthermore, thegraphene layer, besides providing electrical contact and enhancing themodulation range, help to minimize the insertion losses that the devicewould have if a metal contact had been used instead in a plasmonicconfiguration. The leakage current (FIG. 8(d)) confirms the goodness ofthe capacitor and the Al₂O₃ layer and ensure a correct operation whenmodulating the optical signal (FIG. 8(e)).

It is noted that the invention is shown and described for lightmodulation. It is further noted that the modulator concept can also beused in free-space technology such as for spatial light modulators forexample. These devices modulate either the transmission or reflection ofan optical beam via changing the optical refractive index as done here.In addition, the present invention has emerging applications in quantumphotonics, neuromorphic photonics, and beam steering; enabling use inapplications such as compact phase shifters, nonlinear activationfunctions in photonic neural networks, phased array applications forLiDAR, etc. The invention can avail ease of fabrication and potentialCMOS integration.

The foregoing description and drawings should be considered asillustrative only of the principles of the invention. The invention maybe configured in a variety of shapes and sizes and is not intended to belimited by the embodiment. Numerous applications of the invention willreadily occur to those skilled in the art. Therefore, it is not desiredto limit the invention to the specific examples disclosed or the exactconstruction and operation shown and described. Rather, all suitablemodifications and equivalents may be resorted to, falling within thescope of the invention.

1. A photonic waveguide assembly comprising: a photonic waveguide fortransmission of light or a substrate; and an optical refractive indexmodulator positioned about said photonic waveguide or substrate tomodulate the phase or amplitude, or combination thereof of the lighttraveling in said photonic waveguide, or transmitting through orreflecting of said substrate.
 2. The assembly of claim 1, wherein thelight has an index of refraction and said optical refractive indexmodulator alters the index of refraction of the light traveling in saidphotonic waveguide, or transmitting through or reflecting of saidsubstrate.
 3. The assembly of claim 1, wherein said optical refractiveindex modulator comprises a Transparent Conducting Oxide (TCO).
 4. Theassembly of claim 3, wherein said TCO comprises Indium Tin Oxide (ITO).5. The assembly of claim 3, wherein said TCO comprises Indium Tin Oxide(ITO), Indium-doped Zinc Oxide (IZO), Gallium-doped Zinc Oxide (GZO),Aluminum doped Zinc Oxide (AZO), Fluorine doped Tin Oxide (FTO),Magnesium-doped Zinc Oxide (MZO), Aluminum and Gallium co-doped ZincOxide (AGZO), Indium gallium zinc oxide (IGZO), Indium oxide (In₂O₃) orZinc Oxide (ZnO).
 6. The assembly of claim 1, wherein a) said photonicwaveguide comprises Transverse Electric (TE) or Transverse Magnetic (TM)modes, photonic or surface-plasmon-based mode, or hybrid ofphoton/plasmon modes, and/or b) or said substrate.
 7. The assembly ofclaim 1, wherein said assembly can be integrated into silicon photonics,silicon nitride photonics, III-V based photonics, polymer-basedwaveguide structures, any oxide or nitride based waveguide platform suchas SiO₂ for example, or any other material forming a waveguide includingfiber-based structures and/or transmission/reflection geometry basedsubstrate systems.
 8. The assembly of claim 1, said optical indexmodulator capacitively inducing carrier accumulation/depletion leadingto a change in an optical index of refraction of the light traveling insaid photonic waveguide or substrate which modulates the phase and/oramplitude of the light traveling in said photonic waveguide or structureabove/below the substrates.
 9. The assembly of claim 8, the beamimpeding onto or transmitting through the substrate.
 10. The assembly ofclaim 1, wherein the change in the optical index of refraction can be areal part of the optical index of refraction and/or an imaginary part ofthe optical index of refraction.
 11. The assembly of claim 1, whereinsaid optical index modulator comprises a first TCO capacitor electrode,and further comprising a second capacitor electrode a resistivematerial.
 12. The assembly of claim 11, wherein the resistive materialcan be either a TCO material, metal, a 2D material, dielectric or dopeddielectric, gel-like material or any combination of these materialsthereof.
 13. The assembly of claim 1, wherein said optical refractiveindex modulator is in close proximity to the contacts of said photonicwaveguide or substrate.
 14. The assembly of claim 13, wherein the closeproximity is ≤λ, where λ is the wavelength of light propagating throughthe said waveguide.
 15. The assembly of claim 1, wherein said opticalrefractive index modulator alters the optical refractive index of theactive TCO material by up to Δn_(TCO)˜1.
 16. A photonic Mach ZehnderInterferometer (MZI), comprising: a first arm comprising a firstphotonic waveguide transmitting a first light having a first phase; asecond arm comprising a second photonic waveguide transmitting a secondlight having a second phase; and a Transparent Conducting Oxide (TCO)based assembly positioned about said first photonic waveguide of saidfirst arm to modulate the first phase of the first light traveling insaid first photonic waveguide.
 17. The MZI of claim 16, wherein said TCOcomprises an optical refractive index modulator positioned about saidfirst photonic waveguide to modulate the phase or amplitude, orcombination thereof of the light traveling in said photonic waveguide,or transmitting through or reflecting of said substrate.
 18. The MZI ofclaim 7, wherein the TCO modulates the first phase of the first lighttraveling in said first photonic waveguide with respect to the secondphase of the second light traveling in said second photonic waveguide,so that the first light is either in-phase with or out-of-phase with thesecond light.
 19. The MZI of claim 16, wherein said photonic waveguideassembly or said substrate assembly has half wave voltage and lengthproduct V_(π)L of 63-520 V·μm.
 20. A photonic linear absorptionmodulator, comprising a photonic waveguide or substrate transmitting alight with an amplitude and a Transparent Conducting Oxide (TCO) basedassembly positioned about said photonic waveguide or substrate tomodulate absorption of the light traveling in said photonic waveguide orsubstrate resulting in different amplitude at an output of saidwaveguide or substrate due to changing absorption of the TCO basedassembly with bias.
 21. A photonic crystal cavity modulator, comprisinga photonic crystal waveguide or substrate transmitting a light and aTransparent Conducting Oxide (TCO) based assembly positioned about saidphotonic crystal waveguide or substrate to modulate a phase of the lighttraveling in said photonic crystal waveguide or substrate causingresonance shift of the cavity.
 22. The MZI or linear absorptionmodulator or photonic crystal cavity of claim 16, wherein said TCO formsa first capacitor electrode, and further comprising a second capacitorelectrode with a resistive material as either a TCO material, metal, a2D material, dielectric or doped dielectric, gel-like material or anycombination of these materials thereof, said first and second capacitorelectrodes arranged vertically or laterally.
 23. An electro-opticcapacitor comprising: a first electrode comprising a TransparentConducting Oxide (TCO); and a second electrode comprising a2-dimensional material forming the second electrode.
 24. The capacitorof claim 23, where the 2-dimensional material comprises graphene.